The present invention relates generally to radio frequency electrodes for tissue ablation, and more particularly to an improved RF electrode having a spreading array of wires to ablate large volumes of tissue.
The liver is a common repository for metastasis from many cancers, including those of the stomach, bowel, pancreas, kidney, and lung. In colorectal cancer the liver is the initial site of spread in more than one-third of patients, and is involved in more than two-thirds at the time of death. While patients with untreated colorectal metastasis to the liver have no five year survival, patients undergoing surgical resection have approximately a 25-30% five year survival. Unfortunately, only a limited number of patients are candidates for surgical resection.
Cryosurgery is also used for the treatment of hepatic metastasis. Cryosurgery, which relies on a freeze-thaw process to nonselectively kill cells, has been found equally effective as surgical resection but is more tissue sparing. While an improvement over open surgical tissue resection, cryosurgery still suffers from disadvantages. It is an open surgical procedure, requires placement of up to five relatively large probes, and can only be applied to a limited number of lesions. While percutaneous probes are being developed, they are currently capable only of treatment of smaller lesions. Typical lesions common to colorectal metastasis, however, are relatively large. Therefore, the outlook for percutaneous cryotherapy is guarded.
A number of investigators have used radio frequency hyperthermia with placement of external electrodes, for the treatment of liver cancers. Tumor cells are known to be more sensitive to heat than normal cells, and externally applied regional hyperthermia delivered with radio frequency tends to ablate the tumor while sparing the normal tissue of significant damage. While this therapy improves the response to systemic chemotherapy, it has uncertain benefit for long-term survival. One limitation of hyperthermia is that it is difficult to heat the tumors to a lethally high temperature. Moreover, tumor cells tend to become thermoresistant if they survive early treatments.
Percutaneous laser hyperthermia has also been used for primary and metastatic liver cancer. Laser fibers are introduced through needles, under ultrasound guidance. The lesions generated by laser are represented by hyperechoic foci on the real time ultrasound images, which can be used to monitor the size of the lesion. Low energy single fiber systems, which do not require a cooling system along the fiber, can generate areas of necrosis limited to approximately 15 mm diameter. Such small diameters are insufficient for the vast majority of lesions encountered clinically thus requiring multiple fiber placement and prolonged procedure times.
Radio frequency (RF) hyperthermia, using a standard electrosurgical generator and a fine needle partially sheathed in plastic, has also been proposed for the treatment of liver and other solid tumors. In one system, the apparatus was capable of generating lesions of approximately 1xc3x972 cm in a pig liver. In order to produce larger treatment volumes with a single needle, high currents and temperatures have been employed, but produce charred and carbonized tissue, without enlarging the tissue volume being treated. To treat a larger lesion, multiple needle passes in different locations would be needed. In preliminary testing, this system established a 75% survival at 40 months.
It can therefore be seen that the treatment of primary and metastatic liver tumors and other solid tumors elsewhere in the body, remains problematic. Surgery is effective, but only a small percentage of affected patients are candidates. Cryotherapy has had improved results; but its applicable patient population is essentially the same as that for surgery. The percutaneous methods have the virtue of being less invasive, so they can be appropriately used for a larger spectrum of patients, but current percutaneous methods all suffer from a limited ability to ablate a large volume of tissue in a single procedure with a single probe passage.
It is therefore a general object of the present invention to provide an improved electrosurgical method and probe deployable in a percutaneous procedure that will produce a large volume of thermally ablated tissue with a single deployment.
It is a further object that such methods and probes should be useful in open surgical as well as percutaneous procedures.
Another object is to provide an electrosurgical probe which will provide uniformly treated tissue within a large volumetric lesion.
Still another object of the present invention is to provide a percutaneous electrosurgical probe which requires only a small access hole but provides for large volumetric tissue ablation.
Still another object is to provide an electrosurgical probe which avoids the problems of charring and carbonization common with single needle probes.
These and other objects will be apparent to those skilled in the art.
The present invention provides both methods and apparatus for the radio frequency (RF) treatment of a specific region within solid tissue, referred to hereinafter as a xe2x80x9ctreatment region.xe2x80x9d The methods and apparatus rely on introducing at least two electrodes, and usually at least three electrodes to a target site within the treatment region. After reaching the target site, the plurality of electrodes are deployed within the solid tissue, usually in a three-dimensional array and preferably in a configuration which conforms to or encompasses the entire volume of the treatment region, or as large a portion of the volume of the treatment region as possible. More preferably, the adjacent electrodes are evenly spaced-apart from each other (i.e., pairs of adjacent electrodes, will be spaced-apart in repeating pattern) so that application of RF current through the electrodes will result in generally uniform heating and necrosis of the entire tissue volume being treated. Advantageously, the use of multiple electrodes to treat a relatively large tissue volume allows the RF energy to be applied with a lower current density (i.e., from a larger total electrode area) and therefore at a lower temperature in the tissue immediately surrounding the electrode. Thus, charring and carbonization of tissue (which has heretofore been associated with the use of single electrode systems) is reduced. The uniform treatment of a large volume of tissue reduces the number of electrode deployments which are necessary for treating a tissue region of any given size.
In a first particular aspect, the method of the present invention comprises introducing at least two electrodes through solid tissue to a target site within a treatment region. The at least two electrodes are maintained in a radially constrained or collapsed configuration as they are advanced through the tissue to the target site and are then deployed from the target site further into the treatment region in a desired divergent pattern. RF current flow is then established between the at least two electrodes (i.e., bipolar) or among at least the two electrodes and a separate return electrode (i.e., monopolar). The monopolar return electrode will have a surface area which is sufficiently large to dissipate any electrosurgical effect. The at least two electrodes may be deployed by a variety of specific techniques. For example, a sheath may be initially placed using an obturator or stylet to the target site in a conventional manner. After removing the obturator or stylet, the electrodes can be introduced through the sheath and advanced from the distal end of the sheath into the solid tissue. Optionally, the electrodes may be disposed in or on an elongate member, such as a tube which reciprocatably receives the electrodes. The electrodes may then be advanced from the tube, or alternatively the tube may be withdrawn proximally from over the electrodes prior to advancement of the electrodes from the sheath into the tissue.
In a second specific aspect, the method of the present invention comprises advancing at least three electrodes from a target site within the treatment region. The electrodes diverge in a three-dimensional pattern, preferably with individual electrodes being evenly spaced-apart to provide for uniform volumetric treatment, as discussed above. Treatment is then performed by passing RF current among the at least three electrodes or between said three electrodes and a return electrode. Preferably, the method will employ more than three electrodes, often deploying at least five electrodes, preferably employing at least six electrodes, frequently employing at least eight electrodes, and often employing at least ten electrodes or more. It will be appreciated that a larger number of individual electrodes can enhance the uniformity of treatment while limiting the amount of power (current density) emitted from any single electrode, thus reducing the temperature in the immediate region of the electrode(s). Optionally, the at least three electrodes may be everted, i.e., turned first in a radially outward direction and then in a generally proximal direction, as they are advanced from the target site. The use of such multiple, everted electrodes provides a preferred array for treating relatively large tissue volumes. In particular, arrays of everted electrodes will provide current and heating in generally spherical volumes which will more closely match the spherical or ellipsoidal geometries of the typical tumor or other lesion to be treated. In contrast non-everted electrode arrays will often effect a conical or irregular treatment volume which may have less widespread applicability.
In a first aspect of the apparatus of the present invention, a probe system comprises an elongate member having a proximal end and a distal end. At least two solid-tissue-penetrating electrode elements are reciprocatably disposed on or in the elongate member so that they may be advanced into tissue after the elongate member has been introduced through solid tissue to a target site in or near the treatment region. A means for introducing the elongate member through tissue to the target site is also provided. The means may take a variety of forms, including a sheath and obturator (stylet) assembly which may be used to provide the initial penetration. Alternatively, a self-penetrating element may be provided directly on the elongate member. Other conventional devices and techniques of the type used for introducing shafts and other elongate members to solid tissue may also be employed.
The tissue-penetrating electrode elements may comprise wires which are received within an axial lumen of the elongate member. For example, the wires may be bundled together over a proximal portion thereof, but remain separate and shaped over their distal portion so that they diverge in a selected pattern when advanced into tissue. Usually, the wires will be advanced directly from the elongate member (when the elongate member is left inside the sheath or the sheath is withdrawn), but could alternatively be advanced from the sheath when the elongate member is withdrawn proximally from over the electrodes prior to penetration of the electrodes into the tissue.
In a second aspect of the apparatus of the present invention, a probe system comprises an elongate member having a proximal end and a distal end, and at least three solid-tissue penetrating electrode elements reciprocatably attached to the elongate member. The at least three electrodes are configured to diverge in a three-dimensional pattern as they are advanced in a distal direction from the elongate member. Usually, the elongate member is a tube having an axial lumen which reciprocatably receives the tissue-penetrating electrode element, and the electrode elements comprise individual wires which may be bundled as described above. The distal ends of the wires or other electrode elements are preferably shaped so that they will assume a radially constrained configuration while present in the axial lumen of the tube and will assume a radially divergent configuration when axially extended from the tube. In a preferred configuration, the distal ends of at least some of the wires are shaped so that they assume outwardly everted configuration as they are axially extended from the tube or other elongate member. The probe system may include one, two, or more groups of at least three electrodes which are axially spaced-apart from each other. In particular, such axially spaced-apart groups of electrodes may extend from the distal end of the elongate member or may be distributed along the elongate member and individually extendable to assume the desired three-dimensional configuration. Preferably, each group of tissue-penetrating wires or other electrode elements will include more than three electrodes, as described generally above.